1. Technical Field
The present invention relates to liquid crystal display devices, and in particular to a liquid crystal display device having a blue phase liquid crystal layer.
2. Related Art
Liquid crystal display devices have become the mainstream of development of display devices in recent years due to the advantages of low operation voltage, no radiation scattering, light weight and small size. Conventional liquid crystal display devices usually employ nematic crystal to change polarizing direction or status of the passing light, and employ a polarizer plate to convert the different polarizing status into bright/dark contrast for displaying; and thus the response time is relatively long.
On the other hand, response time of blue phase liquid crystal material is short, and thus the blue phase liquid crystal material begins to receive attention. Blue phase is liquid crystal phase between isotropic and cholesteric phase, and itself can be sorted into three phases: the first blue phase BPI, the second blue phase BPII, and the third blue phase BPIII. The third blue phase which has highest existing temperature is amorphous, and the first and second blue phases have cubic structure. Referring to FIGS. 1a to 1d, FIGS. 1a and 1b respectively illustrate the crystal lattice structure and disclination line of the first blue phase, and FIGS. 1c and 1d respectively illustrate the crystal lattice structure and disclination line of the second blue phase.
As shown in FIGS. 1a and 1b, the base unit of the lattice structure of the first blue phase and the second blue phase is double twist cylinder (DTC) 10. The double twist cylinders 10 are perpendicularly arranged in the space. In comparison, the first blue phase liquid crystal has a body-centered cubic (BCC) structure and the second blue phase liquid crystal has a simple cubic (SC) structure. The disclination line of the first blue phase liquid crystal and the second blue phase liquid crystal are shown in FIGS. 1c and 1d. Differing from nematic, smectic, isotropic and other liquid crystal phase, many colored platelet texture can be observed from the first blue phase liquid crystal and the second blue phase liquid crystal with polarizing microscope.
However, blue phase liquid crystal is unstable, and thus can only exist in a very narrow temperature range having a width of about 1° C. Therefore, blue phase liquid crystal can not be easily applied in a common apparatus. To increase the operation temperature range of the blue phase liquid crystal, conventional methods include adding reactive monomer or forming light curable polymer stabilized blue phase liquid crystal by changing the ingredient for synthesizing thereof, which can stabilize the blue phase.
Referring to FIG. 2, which is a schematic view showing a conventional method for forming polymer stabilized blue phase liquid crystal. As shown in FIG. 2, first of all, the blue phase liquid crystal 30 is disposed between a thin film transistor array substrate 22 and a color filter substrate 24. Compared with the optically anisotropic characteristic of common nematic and smectic liquid crystal, the blue phase liquid crystal 30 is optically isotropic prior to the polymer stabilized blue phase liquid crystal is formed. In other words, the blue phase liquid crystal 30 almost has no the characteristic of birefringence. But at this time, the blue phase liquid crystal 30 can only exits in a very narrow temperature range, and it is not easy to apply it in to a practical product. To expand the existing temperature range of the blue phase liquid crystal, the conventional methods includes adding a little reactive monomer 28 and photoinitiator 26 into the blue phase liquid crystal liquid 30, and irradiating the liquid crystal liquid 30 with an ultraviolet light at the existing temperature range to perform a polymerizing reaction such that the macromolecule is polymerized on the disclination line thereby obtaining a polymer stabilized positive blue phase liquid crystal 34 having a gel-like structure. The blue phase of the polymer stabilized blue phase liquid crystal 34 can exist in a temperature range of 60° C.
Ideally, when there is no electric applied the polymer stabilized positive blue phase liquid crystal 34 should demonstrate the characteristic of optically isotropic. However, in practice, conventional polymer stabilized positive blue phase liquid crystal 34 can not demonstrate perfect isotropic property when there is no voltage applied; that is, the birefringence rate is greater than zero (Δn=n//−n⊥>0), and thus usually cause the problem of light leakage in dark state resulting in decreasing of the contrast ratio of display devices. Referring to FIG. 3, which is a schematic view showing a relation between the operation voltage and the transmittance of the conventional polymer stabilized positive blue phase liquid. As shown in FIG. 3, conventional blue phase liquid crystal display devices employ a horizontal electric field to drive the polymer stabilized positive blue phase liquid crystal. When the volts of the voltage applied is equal to zero, in other words, there is no voltage applied to the liquid crystal, the polymer stabilized positive blue phase liquid crystal still has a transmittance of 0.6%, which leads to the problem of light leakage in dark state and the contrast ratio can only reach to 23.